This application relates to techniques and devices for reducing noise in oscillating signals, and more specifically, to techniques and devices for reducing phase noise.
An ideal carrier signal is a signal that oscillates at a single carrier frequency. Information can be imposed on such a single-frequency signal by modulation to produce side bands around the carrier frequency. A modulated carrier signal can then be transmitted to a receiver and is demodulated to recover the information in the side bands.
One common problem in this process is that a carrier signal is usually not so ideal. A carrier signal often has some small side bands that randomly appear around the carrier frequency due to the noise in the carrier generator. These random side bands constitute undesirable phase noise in the carrier and can adversely affect extraction of the useful information from the carrier. In RF or microwave oscillators, electronic components, such as a RF amplifier, often contribute to this phase noise in the carrier signal.
Techniques for reducing such phase noise in RF or microwave oscillator circuits often use a high Q microwave resonator as a frequency discriminator. For example, dielectric resonators and quartz resonators have been used for this purpose.
The present disclosure includes techniques that use an optical delay element and an electrical interferometer to facilitate reduction of phase noise in a carrier generator.
One embodiment is an opto-electronic device that has an oscillator and a noise suppression module. The oscillator is configured to produce an oscillation signal at a carrier frequency and is responsive to an electrical control signal to change a characteristic of said oscillation signal, such as the phase noise. The noise suppression module is coupled to the oscillator to produce the electrical control signal in response to the oscillation signal.
The noise suppression module includes an opto-electronic unit and an electrical interferometer that are coupled to each other. The opto-electronic unit has an optical delay element, such as a fiber delay loop or an optical resonator, and a photodetector to produce a first electrical signal indicative of the oscillation signal from the oscillator. The electrical interferometer causes interference between the first electrical signal and a second electrical signal which is also indicative of the oscillation signal from the oscillator. This interference produces a first output signal representing a destructive interference and a second output signal representing a constructive interference. The noise suppression module uses this first output signal from the electrical interferometer to generate the electrical control signal. This noise suppression module can be configured as an independent device. An oscillator can be coupled to this device to reduce its output phase noise.
In one implementation, the optical delay element receives an optical signal which is modulated to carry the oscillation signal and the associated phase noise. The optical signal transmits through the optical delay element to produce a time delay in the first electrical signal. One effect of this optical time delay is to increase the sensitivity in detecting phase noise. The phase noise is then extracted from the first output of the electrical interferometer by suppressing the oscillation signal to produce an amplitude signal. The amplitude signal is converted into an electrical control signal to modify the operation of the oscillator to reduce the phase noise.
The oscillator may be any electrically controlled oscillator, including an opto-electronic oscillator, When the oscillator is an opto-electronic oscillator, the opto-electronic unit of the noise suppression module can be coupled to an opto-electronic feedback loop in the oscillator. In addition, all elements of such a device can be configured to integrate on a single semiconductor substrate.
These and other aspects and associated advantages will become more apparent in light of the detailed description, the accompanying drawings, and the appended claims.